The invention relates to a reciprocating saw, such as a compass (jigsaw) saw or similar saw, and more particularly, relates to a clamp for a blade used in such saws.
In one prior art blade clamp for saws of this type disclosed in U.S. Pat. No. 5,443,276 and EP 0 693 341 A1, the clamping element has a clamping surface which is in the form of a partial cylinder, that is, is curved in a circular arc shape, the clamping element being rotatable about an axis that is offset with respect to the axis of the cylinder. In this way, by rotating the clamping element by means of an externally accessible operating member, the circular arc-shaped clamping surface can be moved between a position in which it is in clamped engagement with a main lateral surface of the saw blade shank, and a released position, in which it is disengaged from the main lateral surface of the saw blade.
This form of the clamping surface is suitable for clamping the saw blade when the dimensions of the saw blade shank vary only within very narrow limits, and when the form and position of the clamping surface remain substantially unchanged throughout the life of the saw blade clamp. Otherwise, a satisfactory clamping action cannot be obtained, or alternatively the clamped engagement can be so tight that the user is not, or only with considerable difficulty, able to move the clamping element into the release position.
An object of the present invention is to provide a saw with an improved saw blade clamp for clamping saw blade shanks of different dimensions, while keeping a clamping force of unvarying magnitude.
According to one aspect of the present invention, a reciprocating saw comprises a housing with a drive train including a motor, a gear train and a reciprocally drivable output shaft. A saw blade clamp is connected to the output shaft for holding a shank of a blade. The clamp comprises a blade seat with an opening for receiving the blade shank. A hand operable clamping element is pivotal relative to the blade seat about an axis between a releasing position and a clamping position. A spiral clamping surface is formed on the element, extends around the axis for engagement with the third axially extending surface of the saw blade shank and the spiral clamping surface has a constant opening angle.
By using a spiral clamping surface with a constant opening angle, an effective and manageable clamping force is exerted on the shank of the clamped saw blade. For all shank dimensions that lie within the prescribed size range, in the clamping position of the clamping element, the same clamping angle or angle of engagement is obtained between straight lines running through the axis of rotation of the clamping element. One line runs perpendicular to the shank surface at which clamping takes place. The other line extends through the point of contact between spiral clamping surface and the shank surface. The included angle is the clamping angle and it is the same angle for blade shanks of different thicknesses. Having the same clamping angle for all cases ensures that the same clamping force is exerted in each case.
The opening angle of the spiral surface should be chosen to produce a clamping angle of between 4xc2x0 and 12xc2x0 when holding blade shanks of the expected thicknesses. The clamping angle is preferably between 8xc2x0 to 11xc2x0 and most desirably about 10xc2x0. If the clamping angle is less than 4xc2x0, clamping can be such that the user would be virtually unable to release it. If the clamping angle is more than 12xc2x0, there is a risk that the engagement between clamping surface and saw blade shank will not be effective. The clamping surface may simply slide along the saw blade shank as the clamping element is rotated without producing an effective clamping force.
The preferred configuration of the blade includes a blade shank having at least three axially extending surfaces and rear end. The seat preferably comprises (a) at least two bearing surfaces adjacent to two of the axially extending surfaces and (b) a stop surface for the rear end.
The clamping element is preferably in the form of a two-armed lever. The clamping surface is formed on one of the arms. An operating arm for manually displacing the clamping element is formed on the other arm.
In an especially preferred practical form, the clamping surface engages a narrow side of the inserted saw blade shank. At the same time, it has been shown that saw blade shanks of a width between 5.8 mm and 7.2 mm can be clamped properly and uniformly without difficulty by means of the spiral clamping element constructed according to the invention.
When the clamped engagement is effected at the narrow side of the inserted saw blade shank, it is advantageous to mold a groove of constant depth into the clamping surface to engage the saw blade shank. With a constant depth of the groove, the effective form of the spiral is not changed, yet the groove provides additional positioning of the saw blade in the clamped state.
The bearing surfaces of the blade seat preferably form part of a receiving slot open towards the side of the clamping surface.
The clamping element can be spring-loaded towards the clamping position, so that when released it will always be urged into this clamping position, thus ensuring a stable operating state. The spring force acts to increase the clamping force. But in relation to the clamping force effective in operation, the increase in clamping force is so small that it does not appreciably influence the clamping force. On the contrary, the clamping force is caused in practice only by the engagement between the spiral clamping surface and the shank of the saw blade, and by the tensile force that acts on the saw blade in operation.
According to a second aspect of the present invention, a saw blade clamp for a reciprocating saw comprises a clamp as described in accordance with the first aspect of the invention.